Poxviruses, and in particular vaccinia virus, have been utilized as systems for the expression of proteins in eukaryotic cells. The capacity to incorporate large amounts of DNA coupled with their wide host range allows for the expression and correct processing of a great variety of proteins in many cell lines. Hybrid viral genomes have been constructed which express bacteriophage RNA polymerase genes that produce high levels of any gene located behind the bacteriophage promoter. We have developed a conditionally lethal vaccinia virus which expresses the T7 bacteriophage RNA polymerase. Under the restrictive conditions of elevated temperature the virus undergoes an abortive infection where greatly reduced amounts of the late or post-replicative genes are synthesized. This allows the expression of the target gene to occur in an environment devoid of late viral gene expression. We are presently quantitating the absolute level of expression of foreign genes in this system and comparing its utility to presently available vaccinia T7 expression systems. We are presently engaged in the construction and evaluation of poxvirus recombinants to develop efficacious and safe expression vectors.